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For long spans the loading can rise considerably. Not only does the load rise in proportion to length, the resulting peak moment on the resisting member rises with the square of length: The section of the member needs to get increasingly large to resist the increased moment and still the designers typically allow a deflection in proportion to length. As I understand it, this is typically L/325. That means that a lintel for a 3300mm opening will allow a 10mm deflection at the limit. Cavity lintels for large spans (e.g. >2400mm) start to become very expensive if there are roof or floor loads near by. For example, a 3750mm lintel with a total UDL capacity of 50kN runs to around £200-£400. Steel beams have huge moment resisting capability depending on their section. There is a range of Universal Beams (aka RSJ) which have a width of 102mm well suited to a 100mm internal leaf. A 3800mm 305x102x25kg beam costs around £200. I want to understand when to swap to such a steel. Considering just universal beams for a 3400mm opening with 50kN UDL: Section Iy(cm4) Deflection Saving 178x102x19kg 1356 L/364 (9.3mm) 22% 203x102x23kg 2105 L/564 (6.0mm) 6% 305x102x25kg 4455 L/1200 (2.8mm) 0% NOTE: 254x102 UB omitted A cavity lintel would probably have a deflection of 10.5mm (L/325). Steel beam advantages: reduced deflection, greater capacity, lower cost Steel beam disadvantage: rarely galvanised and so needs careful painting etc when exposed to cavity, needs boxing in for fire protection Cavity Lintel advantages: galvanised, makes provision for outer leaf, stated capacity so no need for Structural Engineer's input Ease of installation can go both ways. At length, both are heavy. What experience do people have and when do they typically change from cavity lintels to steel beams?

Anybody have an opinion on this method fixing posi-joists to steel? I thought I would have to use a packer in the flange of the steel but seems not.